This invention relates to incandescent lamps of various types which are to be used in a series circuit, such as decorative string sets for holiday lighting or even an incandescent lamp which has been electrically connected in series with some other electrical device wherein a resistance characteristic of said incandescent lamp in the particular electrical circuit is desired to be maintained even after the incandescent lamp filament has failed. More particularly, the present invention pertains to a novel internal shunt construction for these type incandescent lamps.
Internal shunt constructions for incandescent lamps are already known and most commonly used in holiday lighting strings electrically connected in a series circuit. One type shunt employs a number of turns of fine anodized aluminum wire wound about the inner lead-in wires of the lamp wherein the anodized oxide coating acts as an electrical insulator. If the incandescent lamp filament should fail, then the supply voltage to the lamp is impressed across the shunt member which causes dielectric breakdown of the oxide coating along the shunt member to operate as an electrical conductor and preserving the resistance characteristics of the incandescent lamp. The shunt member is designed so that the breakdown voltage of the oxide coating is lower than the supply voltage to the incandescent lamp. Understandably, wide variation in the breakdown voltage is undesirable since the lamps cannot be replaced in many integral string sets. Another significant problem arises with fluctuation in the electrical resistance characteristics of this type shunt construction which can undesirably influence operation of other electrical devices in the series circuit. An improved shunt construction demonstrating more uniformity in the dielectric breakdown voltage and electrical resistance characteristics is disclosed in Pat. Appln. Ser. No. 859,056, filed Dec. 9, 1977, in the name of R. L. Hickok, and assigned to the present assignee, which utilizes two strips of anodized aluminum foil that are fastened together around the inner lead-in wires of an incandescent lamp. In said shunt construction, the glass bead member used to support and locate the inner lead-in wires can be eliminated and the shunt member may comprise a single long strip folded in half about the lead-in wires. A more reliable shunt is obtained in this manner, and the shunt member itself can be easily manufactured automatically.
Mechanical placement of both above type shunt members in the proper location within the lamp device has also proven to be a difficult procedure. Unless both types shunt members are properly secured between the lamp inleads, there can also be unreliable shunt operation as well as possible interference with the normal lamp operation before the filament burns out. If the above identified foil shunt construction is used to replace the conventional glass bead support which spaces apart the lamp inleads, normal lamp operation could cease if the foil member becomes unfastened. Additionally, normal manufacturing variation in the voltage breakdown and electrical resistance characteristics of both these type shunt members is encountered which can have an adverse effect upon the shunt operation.
It would be desirable, therefore, to provide a still further improved shunt member construction which is less subject to all of the foregoing indicated difficulties. It would also be desirable to provide such improved shunt member construction requiring little modification in the otherwise conventional lamp manufacture.